The present invention relates generally to glass hermetically sealed ceramic-metal composite bodies and a method of producing the composite bodies. More particularly, the present invention relates to composite bodies of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 glass ceramic bonded within a generally cylindrical aluminum containing austenitic stainless steel member and a superior method of hermetically sealing the glass ceramic to the metallic member. The government has rights in this invention pursuant to Contract No. DE-AC04-76DP00053 awarded by the U.S. Department of Energy to EG & G Mound Applied Technologies (formerly Monsanto Research Corp.)
In the manufacture of glass ceramic-metal composite bodies, it is necessary that the thermal expansion coefficient of the glass ceramic substantially match that of the metal member, such that when the composite body is cooled during manufacturing, the hermetic seal between the glass ceramic and metal will not crack due to stresses resulting from differential thermal expansion. Ideally, the hermetic sealed composite body, particularly the seal portion thereof, should be under a slight compression after cooling, such that the metal member is tightly fixed around and bonded to the glass ceramic. To achieve the desired thermal expansion coefficient matching, it is necessary to substantially crystallize the glass ceramic, which is accomplished by a crystallization heat treatment. Substantial crystallization of the glass ceramic also provides the composite body, particularly the glass ceramic portion thereof, with high resistance to chemical attack.
Additionally, the metal member should be compatible with the glass ceramic during the sealing operation, such that no adverse reactions occur at the hermetic sealing interface which would otherwise deteriorate the seal between the metal member and the glass ceramic.
It has been known to seal lithia-alumina-silica glass ceramic to Inconel 718 (hereinafter IN-718), which is essentially a nickel-chromium alloy. However, various drawbacks have been encountered in such composite bodies as well in the manufacture thereof.
For instance, during the sealing process and before the glass ceramic is substantially crystallized (i.e., when the material is still in a glassy state), chromium and iron migrate from the IN-718 member into the LAS glass. Such migration causes phosphorus in the glass to react with the chromium and iron to produce metal phosphides at the hermetic sealing interface. Such reactions at the interface cause the nucleating agents in the glass ceramic at the interface portion to decrease, and thus result in a reduction of crystallinity of the glass ceramic at the interface. The decreased crystallinity may substantially lower the thermal expansion coefficient at the interface, and thus, may cause a composite body including such an interfacial region to crack when cooled during manufacturing, due to differential thermal expansion.
Further, IN-718 composite bodies have another drawback in that they are not easily laser weldable. As is well known in the art, it is highly desiraable that the metal members utilized in such composite bodies are easily laser weldable to other metals as well as to each other. Lastly, since IN-718 alloy is primarily nickel based, it is relatively expensive.
Glass ceramic-metal composite bodies have also employed LAS glass ceramic in combination with the 300-series austenitic stainless steels. A frequently utilized 300-series alloy is 304-L and has the following composition: 18-20 wt% Cr, 8-12 wt% Ni, 0.03 wt% C, 2.0 wt% Mn, 1.0 wt% Si, and the remainder being Fe. However, as with the IN-718 alloys, these metals also suffer various drawbacks. For instance, the thermal expansion coefficients of the 300-series alloys are generally much higher than that of the LAS glass ceramic and thus, cracking at the hermetic interface often results during manufacture of these composite bodies. Further, the 300-series alloys also experience adverse reactions at the interface between the alloy member and the LAS glass ceramic. Furthermore, glass devitrification cycles utilized to obtain proper properties of the LAS glass ceramic degrade the tensile strength of the 300-series alloys to almost 50% that of its initial strength. These alloys cannot be strengthened by subsequent heat treatments and thus, the low strength which results after the glass devitrification cycle permanently remains in the composite body.